1. Field of the Invention
The invention relates to a process for preparing isocyanatoorganosilanes and to certain isocyanatoorganosilanes.
2. Description of the Related Art
For some time there has been great interest in an economic method for preparing isocyanatoorganosilanes in high yields and purities. These compounds are of great economic importance, since they are used, for example, industrially as adhesion promoters between organic polymers and inorganic materials. The compounds are sometimes termed coupling agents or crosslinkers.
For preparation of isocvanatoorganosilanes, processes are preferred in which starting materials pose little or no safety risk, in order to facilitate handling and procedure. In the case of the processes previously used, isocyanatoorganosilanes are prepared in relatively low amounts in expensive, low-efficiency processes.
For example, in the process described in U.S. Pat. No. 6,008,396, carbamatoorganosilanes are converted to the isocyanates in hot inert media with elimination of alcohol. However, this process. can only be operated semi-continuously, since the concentration, of impurities in the medium, even after a short time, increases in such a manner that the purity of the desired product decreases significantly.
In the case of the process described in EP-A-1010704 for preparing isocyanatoorganosilanes, carbamatoorganosilanes are thermally cleaved in the liquid phase to give the corresponding isocyanatoorganosilanes with catalysis by tin(II) chloride. A disadvantage of this process is the highly complex process for isolating and purifying the desired products, which leads to low yields. Therefore, this process has not been employed industrially to date.
The thermal cleavage of carbamatoorganosilanes in the gas phase under atmospheric or reduced pressure is disclosed by EP-A-649850. However, the yields are unsatisfactory.
The invention relates to a process for preparing isocyanatoorganosilane of the general formula 1
R2R3R4Sixe2x80x94R1xe2x80x94Nxe2x95x90Cxe2x95x90Oxe2x80x83xe2x80x83(1),
in which gaseous carbamatoorganosilane of the general formula 2
R2R3R4Sixe2x80x94R1xe2x80x94NHxe2x80x94COxe2x80x94ORxe2x80x83xe2x80x83(2),
where
R is a monovalent C1-C10-alkyl radical,
R1 is a divalent C1-C6 hydrocarbon radical and
R2, R3 and R4 are in each case a methyl, ethyl, n-propyl, isopropyl, methoxy, ethoxy, n-propoxy or isopropoxy radical, is heated to 300xc2x0 C. to 1000xc2x0 C. in the presence of a heterogeneous catalyst. Using the inventive process, high yields and selectivities are achieved.